Well, there are 3 things you can do.
Search eBay for junk yard listings for that part. You'll find them. It probably wouldn't be expensive, and they'll sell them when they're untouched. That is option 1.
Option 2 is to cut the busted plastic off, and bolt a metal L-Bracket that fits in the spot to the vertical tab. Use a bolt that is large enough to secure the part, but small enough that the hole you drill doesn't compromise the plastic. Then use another bolt to connect the diffuser.
Option 3 is the aforementioned zip-tie....
'09 PSM G8 GT - OPG Intake, DOD-Delete w/ Stage 1C 218/229 .600 Lift 117.5LSA, Kooks 1 7/8 Mids, Solo Axle-Back w/ X-Pipe, SuperPro Bushings (Front and Rear). BMR Sway Bars (Red)
"If you want to criticize someone else's work, be sure that your own is in order first."